Refining toluene with toluene sulfonic acid



Aug. l0, 1948. F. 1'. wADswoRrH Erm.

l REFINING TOLUENE WITH TOLUENE SULFONIG ACID Filed Hay 11. 1945 Patented Aug. 1li, 1948 REFINING TOLUENE WITH TOLUENE SULFONIC ACID Francis T. Wadsworth, Texas City, Robert J. Lee, La Marque, and Preston L. Brandt, Galveston, Tex., asslgnors to Pan American Refining Corporation, New York, N. Y., a corporation of Delaware Application May 11, 1945, Serial No. 593,146

This invention relates to a process of refining toluene and more particularly to a process for refining synthetic toluene produced from petroleum by catalytic conversion. One object of the invention is to produce nitration grade toluene from petroleum which will meet all the requirements for the manufacture of trinitrotoluene, particularly the requirements of acid wash color and color stability. Another object of the invention is to refine synthetic toluene with a minimum loss and a minimum consumption of chemical treating agents.

It has heretofore been the practice in manufacturing toluenefrom the by-products of coal distillation to refine the distillates or so-called light oils with strong sulfuric acid after which the distillates were fractionated to produce the desired fractions including a toluene fraction. When properly fractionated and acid treated, toluene made by this process has met the specifications for nitration grade Without great difficulty. However, when toluene is made from petroleum by catalytic aromatization of paraiilnic and cycloparamnic hydrocarbons, followed by a suitable recovery method such as extractive distillation with phenol, solvent extraction or azeotropic distillation, the resulting product is contaminated with olenic, diolefinic and/or paramnic and cycloparaffinic materials which are objectionable and must be substantially removed to produce a nitration grade toluene. Sulfuric acid treating has been used to accomplish the finishing step. Ex-

eessive amounts of strong sulfuric acid have been required to effect this and substantial loss of toluene is thus encountered. It has been particularly diflicult to produce from` petroleum a synthetic toluene which is satisfactory from the standpoint of acid-wash color and acid-wash color stability, and although synthetic toluene refined with heavy sulfuric acid treating alone may be of satisfactory color by direct observation and may even show a satisfactory acid-wash color test when freshly prepared, there is a tendency for it to develop an unsatisfactory acid-wash color lafter storage. The acid-wash color test referred to is commonly used 'in the testing of nitration grade toluene. This test is described in U. S, Army specification 55-11-38C-Section E5, according to which the sample of toluene in question is shaken with 96% sulfuric acid and the Claims. (Cl. 26o-674) amount of color development is noted by comparing the color of the acid with certain color standards. The color obtained in this way is known as the acid wash color.

In the manufacture of toluene synthetically from petroleum, the process most commonly used is substantially as follows: A naphtha fraction o'f" petroleum, generally boiling in the range of about 100 to 250 C., is subjected to hydroforming at temperatures in the range of 900 to 1100 F, in

the presence of an aromatization catalyst and i hydrogen. Suitable catalysts for the purpose are the oxides of sixth group metals, particularly chromium and molybdenum oxides, and certain other oxides such as vanadium oxide, preferably supported on the oxides of other metals, more particularly magnesia and alumina. Pressures of the order of 100 to 400 p. s. i. are preferred and the amount of hydrogen employed is suitably about 1000 to 3000 cubic feet per barrel of naptha treated. Space velocities of 0.5 to 2 are commonly used, depending on the character of the cata- -lyst and the naptha being treated. Hydrogen is usually recycled in the operation.

The hydroformed naptha obtained from this process may contain about 15 to 20 per cent of toluene and this concentration is largely increased by careful fractionation to remove the toluene boiling fraction, generally a fraction boiling in the range of about 90 to 120 C., although better results are usually obtained by selecting narrower boiling fractions, for example a fraction boiling at about 100 to 110 C. This fraction usually will contain a considerable amount, e. g. 25 to 50 per cent, of parain hydrocarbons which are most conveniently removed by solvent distillation, the mixture being distilled in the presence of a suitable polar solvent such as phenol, cresol, etc., by extractive distillation technique. In this operation, the undesired paraffin hydrocarbon is recovered overhead and the toluene is obtained as a bottom with the solvent, from which the toluene is subsequently distilled. .Alternatively the paraflinic contaminant may be removed by azeo- 3 extraction of toluene from the raw product with a solvent such as liquid SO2.

In the further rening of toluene, particularly that produced by hydroforming in this way. we have discovered that liquid phase treating of toluene vapors with a solution of toluene sulfonic acid is particularly effective for removing from the toluene those constituents which are responsible for the undesired acid-wash color and acidwash color instability, and we have devised an apparatus for carrying out the rening process as shown in the accompanying drawing. L

Our process may be carried out with extremely low consumption of the treating agent and with substantially no loss of toluene. Such slight losses of toluene sulfonic acid as do occur are due primarily to decomposition to toluene, SO2 and water, thus avoiding the introduction of hydrocarbon contaminant into the toluene in the finishing step. The use of benzene sulfonic acid and other sulfonie acids than toluene sulfonic acid has been found definitely undesirable because of this unavoidable partial dissociation.

Referring to the drawing, toluene vapor withdrawn directly from the extraction operation by line I or produced in pipe furnace II is injected by line I2 beneath the surface of a pool I3 of liquid, substantially anhydrous toluene sulfonic acid hereinafter referred to as TSA, dissolved in toluene. Preparation of this reagent is described hereinafter. For this purpose, induction line I2 may be perforated in the form of a spreader to distribute the toluene vapor beneath the surface of the pool of TSA. The pool of TSA is conveniently retained within a suitable still or treating vessel I4 to which is connected a vapor outlet I5 and an entrainment arrester I 6 from which vapor line I1 leads to fractionating tower I8 wherein the treated toluene is fractionated to remove from it a small amount of polymer or higher boiling fraction produced in the treating operation, the resulting residue being withdrawn by line I9 while the refined toluene vapor is conducted byline 20 to condenser 2i and thence to separator 22 where water distilled from the treating vessel IB may be collected and recycled by line 23 to the TSA recovery step to be described below. The liquid toluene is removed by line 24 and forced by pump 25 thru mixer 26 wherein it is suitably contacted with an alkaline reagent such as caustic soda solution supplied by line 21 for the purpose of removing traces of acid from the nished product. In settler 20, the alkaline solution is settled out and recycled to the mixer by pump 29, the nished toluene being withdrawn from the process by line 30.

It is preferred to maintain the pool I3 of TSA solution at a temperature in the range of about 100 to 135 C., preferably about 110 to 125 C.

It is an important feature of our process to conmaintained by applying-indirect heat thereto but for our purpose it is more convenient to emPlOy an insulated vessel and supply the heat by means of the stream of toluene vapor introduced thru line I2, the temperature of the toluene vapor being controlled by furnace I0 or by passing the vapor thru a suitable heat exchanger. The toluene vapors, at a temperature of about to 135 C., are introduced into the treater thru line I2, for example at approximately eighteen inches below the surface of the P00101 TSA solution in the vessel. Sufilcient superhcating is maintained in the vapors entering the treate to counteract radiation losses, the amount of superheating being controlled so as to prevent a change in volume ofthe TSA treating solution by condensation or vaporization of liquid toluene from this solution. In a typical installation, the treating vessel I4, oi.' Monel metal or copper, may be approximately four feet in diameter by ten feet long for an installation processing 1000 barrels per day of toluene. Steel vessels may also be used. particularly where a corrosion inhibitor is present such as certain heterocyclic amines, arsenic compounds, etc. 'I'he dimensions of the vessel should be such that the velocity of the vapors disengaging from the liquid is not excessive, since other wise excessive frothing of the treating solution may occur and it is preferred that the upward ve.. locity of the vapors in the vapor space does not exceed about 0.25 to 0.5 foot per second. The entrainment arrester I6 should be used at velocities above about 0.2 foot per second and may be packed with glass wool or suitable ceramic packing offering a large surface to knock back small amounts of liquid or froth which may be carried from the treating vessel. Glass wool packed to a density of about 0.5 pound per cubic foot is satisfactory. Batlles or metallic alloy screens may also be used.

The rate of charging toluene vapor to the treater will vary, depending on the character of the stock treated. When employing a typical synthetic toluene fraction produced by hydroforming of naptha and phenol extraction, having a bromine number of about 0.2 to 0.6, a space velocity of around 200 to 2000 volumes of vapor (at 121 C.) per hour per volume of TSA solution in the treating vessel may be used with TSA concentrations in the treater of about 10 to 30 per cent by weight. Infcommercial practice it is desirable to operate at the higher TSA concentration and at correspondingly higher space velocities. A satisfactory condition is a space velocity of 500 to 1500 and a TSA concentration of about 20 per cent. TSA concentrations in the range of 5 to 50 per cent can be used economically, depending on the character of the toluene charging stock employed. The operation is continuous.

During the treating operations in accordance with our process, the undesirable constituents are chemically converted to various high-boiling products such as polymers, alkymers and other unidentified materials which accumulateV in the treating solution. Surprisingly large concentrations of these high-boiling materials may be allowed to build up in the treating solution without materially affecting the eiliciency of our process. Concentrations high enough to cause the formation of two liquid phases may be used and still obtain effective treating. Charging stocks having less than about 0.5 bromine number can be treated to yields in excess of fifteen barrels of toluene per pound of TSA without fortification. During such operations at least a part of the high-boiling products formed will be carried out of the treating solution and be rejected from the bottom of andere fractionator Il. With charging stocks having lbromine numbers above 0.5 and particularly above 1.0, the accumulation of high-boiling products in the treating solution will become* sumciently rapid to justify fortiication and recovery operations. If tower I8 is omitted, the concentration of high-boiling products in the treating solution must be kept at lower levels, thus necessitating recoveryof TSA from the discarded treating solution. In the case of stocks high in olens, e. g. stocks having bromine numbers above 3 and usually above 1. it is frequently desirable to pretreat 'withv H2804 to reduce the unsaturation before TSA treating as hereinafter described.

This requirement may be expressed another way-if the concentration of TSA in the treating solution required to produce toluene meeting acid wash color specifications is in excess of %,'pretreatment with sulfuric acid will lshow material advantage.

The concentration of high-boiling products in y with water, reinjection of the water solution into the treating zone and rejection of the high-boiling products.

In order to prevent excessive contamination of the reagent, a portion of the reagent is withdrawn continuously or at intervals by pump 32 in line 3S to recovery drum 36'. The reagent is mixed with water which may be introduced by line 35 or the water recovered in separator 22 may be recycled for the purpose` by line 23. If desired, a mixer may be employed for mixing the water with the TSA-toluene solution-before char-ging to the recovery drum 5d.

In drum 3d, stratication occurs with the formation of a layer of TSA in water solution in the bottom of the drum, sufiicient water being employed to give a solution having a. concentration of about to l0 per cent by weight of TSA. This solution is free owing and separates from the hydrocarbon material including toluene which forms an upper layer in the separator S. Coalescers 4may be used if necessary. The TSA in water solution is conducted by line 36 and pump Si back to`the treating vessel It. Fresh TSA is introduced from time to time by line 38 to make up for losses in the process. The amount required, however, is very small in view of the excellent recovery. When charging the aqueous TSA solution to the treater, it is desirable to in` ject it at an even, regulated rate to avoid disturbing the operation of the treater by too great dilution at any one time. The water introduced in this way is rapidly vaporized with the toluene vapors, leaving substantially anhydrous TSA dissolved in toluene.

The toluene and polymerized constituents in the TSA, withdrawn from the treating vessel i3 by line 83 and separated from TSA in 36, are removed from the drum Elfi by pump 39 and line 40 leading to -ractionator I8 where the toluene is recovered and the residual hydrocarbon polymer material is removed from the fractionator by line l as hereinbefore described.

Another method may also be rused to separate TSA from the high-boiling products which accumulate in the treating solution and to permit recovery of TSA for recycle to the treating operation. This method comprises the addition of a paramnic or naphthenic naptha, preferably 4higher boiling than toluene such as heavy 6 alkylate, to a portion of the treating solution which has been withdrawn from the treating zone. The addition of this naptha in the ratio of from 1 to 5 volumes, but preferably 3 to 4 volumes, of naptha per volume of treating solution causes the separation of a lower phase comprising largely anhydrous TSA and containing from to 90 per cent of the total TSA charged to the separation zone. This TSA phase .is recycled to the treater and the hydrocarbon layer is vsent to a rerun tower, suitably fractionator I8, where the toluene is distilled overhead and the high-boiling hydrocarbons and the added diluent are rejected. The naptha used in this recovery step should be sulciently higher boiling than toluene so as to be either retained in the-treater or rejected from the rerun tower.

When employing the hydrocarbon rejection method of recovering TSA, after separating the major portion of the anhydrous TSA and recycling to the treating vessel, the remaining TSA dissolved in the hydrocarbon layer may be recovered by water extraction and the water extract may be separately recycled to the treating vessel as described hereinabove when employing water extraction alone. `The advantage of the combination two-stage recovery with paraffin hydrocarbon in the first stage and water extraction in the second stage lies in a reduction in the amount of water which is employed and which must be evaporated in the treating vessel and decreased corrosion because of thev reduced amount of aqueous TSA solution introduced.

In some cases it is unnecessary to redistill the treated toluene vapors from vessel I6, in which case these are `conducted by by-pass line 4l shown dotted on the drawing leading from entrainment arrester I6 to condenser 2l.

The eiect of the concentration of TSA in the treating vessel in relation to the throughput or space velocity is well illustrated by the following data which were obtained in a continuous treating operation charging raw toluene from l the hydroforming process, maintaining a constant volume of treating agent in the vessel but changing the concentration between experiments by withdrawing a portion of the treating agent and adjusting the temperature of the treater to retain therein suiilcient liquid toluene to provide the proper TSA concentration as indicated. The temperature of the treating solution was about 113 to 120 C. and the space vapor velocity (volume of vapor per hour per volume of agent) was maintained at about 460 V./H./V. The raw toluene charged to the process had an acid wash color of 6 plus and a bromine number of 0.32.

Treated toluene Cone. of Acid Per Cent 'rsi in Wash @mit S53 Bflrmm@ acidity 2 onmiske- Treater Color y o* moved Per cent 32 0 1 0-1 00 52 100 20. 0 0-1 0-1 02 33 93 I5. 5 -'l 0 1 07 15 76 10. 5 0-1 0-1 09 11 69 7. 8 l 1+ 15 7 49 5. 9 1 1 17 7 42 4. 2 1 2 1 2 19 7 35 l 40 Minutes Exposure to Ultraviolet Light 2 Expressed as ppm SO:

The following data show the efect of contact time on treating raw `toluene having a bromine number of 0.15 with a TSA solution of approximately 20 per cent concentration in toluene. The height of the solution above the vapor distributor was 61/4 inches and the treating temperature was about 113 to 115 C. The contact time is expressed as space velocity of the vapors in volume per hour per volume of TSA solution.

Vapor Veloc- Space Velocity Above Brominc No. ity. v./h./v. TSA, Ft. per of Product Bec.

292 048 l8 585 O96 023 876 144 032 l, 170 192 028 l, 461 240 039 l, 754 288 046 2,200 360 040 2, 920 480 041 It will be noted that the bromine number increases progressively with increase in vapor velocity. The acid wash color of the raw toluene was and all samples of treated toluene had an acid wash color of between 0 and 1, and the acid wash color was still 0-1 after twenty-four hours exposure under accelerated test conditions in the Baytown Ordnance Works (B. O. W.) test in which air is bubbled thru a sample at 195 to 200 F. for twenty-four hours and aliquot portions are removed at lfour-hour intervals for acidwash color determination.

The tendency of the TSA-toluene solution to froth during operation has already been referred to. Attempts to prevent frothmg by adding higher boiling aromatic solvents. particularly the alkyl naphthalenes such as methyl naphthalene, were not successful. The velocity of introducing toluene vapor into the pool of treating agent was found to be the critical variable in determining extent of foaming but as previously shown had little effect on quality of treated product over `the range covered. It was found that suspending a layer of glass wool above the level of the pool in the treater effectively served to break the froth and permit the use of higher linear vapor velocities in the disengaging space. Thus, in one operation, using a layer of glass wool of 0.5 pound per cubic foot density spaced two inches above the liquid level in the treater, charging toluene with an injection jet velocity of about 194 feet per second, the vertical velocity of toluene vapor in the dlsengaging space was about 0.36 foot per second and the froth extended into the glass wool layer to a depth of approximately three inches. When no glass wool layer was employed, the height of the froth layer above the pool was eleven inches. The use of a coalescer packed with glass wool or other suitable contacting agent instead of the deentrainer i6 is contemplated, preferably operating the coalescer downflow. Condensate formed in the coalescer may conveniently flow back to the treater by gravity.

The following table shows the results obtained in treating a highly unsaturated raw toluene stock in two stages, sulfuric acid being employed as a pretreat in the first stage and TSA-toluene solution in the second stage. The severity of the preliminary acid treat was varied by varying the amount of acid per barrel of stock. The acid employed was 98 per cent concentration and the TSA solution contained approximately 20 per cent TSA in toluene. The raw toluene stock had a bromine number of 4.86.

l 2 successive 50H1. treats.

It will be noted from these data that the combination treatment using 20 pounds of sulfuric acid per barrel followed by TSA treating gave a product having an acid wash color equal to that obtained when using pounds of sulfuric acid per barrel. Furthermore, the color stabllityof the toluene obtained by the combination treatment was definitely better. Aside from the saving in chemical cost in the use of the smaller amount of acid, there is also a liberal saving in toluene. In the case of another stock having a lower bromine number, which could be treated to specifications with about l5 to 20 pounds of sulfuric acid per barrel, the use of TSA alone resulted in a saving amounting to about 1.5 per cent of the toluene treated. Where the stock has a bromine number above 3, and usually above 1.0, we prefer to apply the preliminary treat with concentrated HzSOi.

The pressure employed in treating toluene according to this process is preferably approximately atmospheric pressure or slightly above, usually only sufficient pressure being applied to the treater to force the toluene vapors thru the subsequent fractionation and condensation zones. For this purpose, pressures of 5 to l5 pounds per square inch gafuge are adequate. The pressure willbe that required to balance the vapor pressure of the solution at the concentration and temperature desired. Thus, when operating at temperatures well above the boiling point of toluene, e. g. to 145 C., the concentration of TSA in the treating solution may be reduced by .increasing the pressure in the treating zone to that equaling the vapor pressure of toluene above the treating solution of the desired concentration. Usually pressures in the range of 5 to 50 pounds per square inch gauge are adequate for this purpose. We may also operate at subatmospherlc pressure at correspondingly lower temperatures.

If desired, the TSA employed' in our process may be manufactured at or near the location of the refining operation by the simple procedure described hereinafter whereby TSA is produced in a convenient form for use ln the process without the necessity of purification, recrystallization, packaging, etc. According to this procedure, toluene is treated with sulfuric acid, either concentrated or fuming, .and the resulting sulfonation product is extracted from the acid with excess toluene. The acid layer is separated from the toluene solution and discarded or used for some other purpose for which it is suitable, for example in the rening of petroleum stocks. The toluene solution containing the TSA may then be pumped directly to the TSA treating plant Without recovery of the TSA in concentrated form. In the case Where it is desired to ship the crude TSA for some distance, it may be concentrated by distillation of part or all of the toluene from the anhydrous TSA or by precipitation of TSA from the toluene by the addition of a suitable paraflinic oil or naptha as hereinabove described.

Having thus described our invention what we claim is:

1. The process of rening synthetic toluene boiling in the range of about 90 to 120 C. obtained by catalytically hydrofor-ming petroleum naptha followed by concentration with a selective solvent of the toluene contained in the hydroformer distillate which comprises contacting the vapors of said synthetic toluene with a toluene solution of toluene sulfonic acid at a temperature in the range of about 100 to 150 C. and a concentration range of about to 50 per cent by weight of toluene sulfonic acid, based on the` weight of said toluene solution.

2. The process of rening crude toluene boiling in the range of about 90 to 120 C. which comprises subjecting it to the action of a solution oi substantially anhydrous toluene sulfonic acid in a pool of liquid toluene, said solution containing about 5 to 50 per cent of toluene sulfonic acid, maintaining the temperature of the treating solution in the range of about 100 to 150 C., withdrawing toluene vapors from said solution and condensing said vapors to produce the refined toluene.

3. The process of claim 2 wherein the temperature of said treating solution is maintained within the range of about 110 to 125 C.

4. The process of refining crude synthetic toluene boiling in the range of about 90 to 120 C. derived from the conversion of petroleum naptha whi-ch comprises injecting the vapors of said toluene below the surface of a pool of treating solution containing about 10 to 30 per cent of substantially anhydrous toluene sulfonic acid dissolved in liquid toluene, maintaining the temperature of said solution within vthe range of about 110 to 150 C., withdrawing toluene vapors from said solution and condensing them to produce the desired refined toluene.

5. The process of claim 4 wherein the toluene vapors are charged to said treating solution at a temperature of about 112 to 135 C. i

6. The process of claim 4 wherein the sai toluene sulfonic acid treating solution is maintained constantly active by withdrawing a portion thereof from said pool, extracting it with water, separating the resulting mixture into an upper toluene phase and a lower aqueous phase containing the toluene sulfonic acid in aqueous solution and returning the aqueous toluene sulfonic acid solution to the treating operation wherein water is evaporated leaving substantially anhydrous toluene sulfonic acid fully restored in the treating zone.

7. 'I'he process of producing/nitration grade toluene from crude toluene boiling in the range of about 100 to 110 C. obtained from the hydroorming of petroleum naphtha which comprises introducing it into a treating zone containing a pool of toluene sulfonic acid dissolved in liquid toluene, maintaining the temperature of the treating zone within the range of about 110 to 150o C., withdrawing vapors of toluene fromv said treating zone to a fractionating Zone, removing fractionated toluene vapors from said fractionating zone and condensing them to produce the desired nitration grade toluene, maintaining the activity of the toluene sulfonic acid treating'agent within said treating zone by withdrawing a portion thereof to a hydrolyzing zone, mixing water with said portion of treating agent in said hydrolyzing zone and separating the resulting hydrolyzed treating lagent into an upper toluene layer and a lower layer of toluene sulfonic acid in water solution, returning the water solution `of toluene sulfonic acid to the treating zone and charging said separated toluene solution from said hydrolyzing zone to said fractionating zone for recovery of toluene therefrom.

8. The process of claim 7 wherein the amount of water employed in said hydrolyzing zone is sufncient to produce an aqueous toluene sulfonic acid solution of about 40 to '70 per cent by weight,

9. The process of claim '7 wherein toluene is charged to said treating zone at a space vapor velocity of about 200 to 2000 volumes of vapor per hour per volume of treating solution.

10. The process of rening toluene to meet the requirements of nitration grade where said toluene is derived from the conversion of petroleum naphtha and is characterized by a boiling point within the range of about to 120 C. and a bromine number greater than 1, comprising initially treating with concentrated sulfuric acid suflicient to reduce the bromine number to less than 1, then completing the refining by contacting the toluene vapors with a toluene solution of toluene sulfonic acid at a temperature of -150 C, under conditions suicient to produce a product having the desired acid Wash color and color stability.

FRANCIS T. WADSWORTH. ROBERT J. LEE. PRESTON L. BRANDT.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Germany Jan. l5, 1924 OTHER REFERENCES Whitehead, Benzol," D. Van Nostrand Co. (1920), New York City, page 130. 

